import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
		}
		sc.close();

		Set<Integer> set = new HashSet<>();
		set.add(a[0]);
		for (int i = 1; i < n - 1; i++) {
			if (a[i] != a[i - 1] && set.contains(a[i])) {
				for (int j = i + 1; j < n; j++) {
					if (a[i] != a[j]) {
						System.out.println(-1);
						return;
					}
				}
				System.out.println(1);
				return;
			}
			set.add(a[i]);
		}
		if (a[0] == a[n - 1] && set.size() > 1) {
			System.out.println(1);
		} else {
			System.out.println(0);
		}
	}
}